A power amplifier is essentially the last stage of a transmitter before a connection to a load, which in a radio transmitter means a transmitting antenna. Preceding stages of the transmitter produce a radio frequency signal by modulating a radio frequency carrier with a lower-frequency payload signal. The task of the power amplifier is to amplify the produced radio frequency signal enough so that directing it in amplified form to a transmitting antenna will cause the radio frequency signal to be transmitted in the form of electromagnetic radiation at a desired power level. The transmitting process should be accomplished as effectively as possible, i.e. with as little loss as possible, especially in portable radio devices where electric energy should be carefully saved in order to prolong the time before next required battery recharging.
One of the principal issues affecting transmission efficiency is impedance matching between the power amplifier and the antenna. Under steady-state conditions a transmitting antenna has a constant input impedance, so it should be rather straightforward to design the output of the power amplifier so that it matches the impedance of the antenna and impedance matching would thus be close to perfect. Problems arise because a practical antenna is not operating under steady-state conditions. Large and/or well-conducting bodies brought close to the antenna change the impedance characteristics of the antenna, which causes impedance mismatch between the antenna and the power amplifier, commonly referred to as antenna mismatch. The hand of a user is frequently close enough to the antenna of a portable telephone to cause problems of this kind. Additionally the impedance of any signal port is a function of frequency and power. Most modern portable radio devices employ transmission power control, which causes the amplification factor, and subsequently also the output impedance, of the power amplifier to change. This again tends to give rise to mismatch problems.
Increased transmission loss is not the only negative consequence of impedance mismatch between a power amplifier and an antenna. Mismatch causes the power amplifier to operate at a needlessly high power level, which tends to cause distortion in the radio frequency signal. The problem is worst with linear amplifiers, the use of which is mandatory with most amplitude-affecting modulation methods, because the amplification level of a linear amplifier cannot be cut back controllably as a response to a detected antenna mismatch as with nonlinear amplifiers. It should be noted, though, that linearity problems caused by antenna mismatch are most prominent at high transmission power levels, and consequently their severity in low-power handheld devices is quite modest.
In principle the same problems arise in every transmission application, regardless of whether the power amplifier is coupled to an antenna, to a transmission wireline or to any other load. Portable radio transmitter applications where the load is an antenna are, however, the most prone to mismatch effects because there it is the most difficult to predict or eliminate the incidentally occurring circumstances that alter the load impedance.
The conventional way of eliminating the effects of antenna mismatch is to use an isolator between the power amplifier and the antenna. The isolator approach is, however, ill suited for small-sized portable radio devices, because a conventional isolator is a large and clumsy component and it draws a prohibitively large electric power during times when it has to compensate a large mismatch. Certain small-sized isolator solutions have been introduced, but it is still unprobable that they could be e.g. integrated with a power amplifier. Additionally the characteristically high power dissipation during severe mismatch conditions remains as a drawback of even small-sized isolators, because the functional principle of an isolator comprises dissipating reflected power in a resistor.
Other known ways exist too for eliminating the effects of antenna mismatch. The patent publication U.S. Pat. No. 5,564,086 presents one functional principle, which is also illustrated in FIG. 1. According to this principle a directional coupler 101 between the power amplifier 102 and the antenna 103 is used to detect antenna mismatch. A processor 104 is coupled to receive the mismatch detection information from the directional coupler 101. Between the power amplifier 102 and the directional coupler 101 there is a controllable variable matching network 105. From an output of the processor 104 there is a connection to a control input of the variable matching network 105. The processor 104 is programmed to respond to mismatch detection information from the directional coupler 101 by changing the characteristics of the variable matching network 105 appropriately, thus reducing the effects of antenna mismatch.
The drawbacks of the arrangement of FIG. 1 are related to the discrete nature of a directional coupler. It is very difficult, if not impossible, to integrate a directional coupler to a common integrated circuit with anything, particularly with a power amplifier. Thus the use of a directional coupled tends to increase overall size and manufacturing cost. Additionally every additional component, even a directional coupler, along the high power signal line from the power amplifier to the antenna carries a risk of increasing losses.